The present invention relates generally to fluid driven piston assemblies, and more particularly to rate shaped fluid driven piston assemblies utilized in hydraulically actuated fuel injectors.
In one class of fuel injectors, a hydraulically driven piston assembly is utilized to raise fuel pressure to injection levels before and during an injection event. In a typical example, a relatively large diameter piston is acted upon by working fluid pressure to drive a relatively small diameter plunger that acts upon fuel to pressurize the same. Since the piston has a relatively large diameter compared to the plunger, these hydraulically actuated fuel injectors are considered to be pressure intensified systems since the fuel pressure is raised to many times that of the working fluid pressure because of the differing hydraulic surface areas. Thus, in these devices, the fuel injection pressure corresponds generally to the area ratio between the plunger and piston, and the pressure of the working fluid acting on the piston. While hydraulically actuated fuel injectors of this type have performed well for many years, engineers are constantly looking for ways to improve the same.
Over the years, engineers have discovered that emissions can be significantly reduced at certain operating conditions by providing a particular injection rate shape. In many cases, emissions can be improved when the initial injection rate is controllable, and when there is a nearly vertical abrupt end to an injection event. One strategy for introducing front end rate shaping into hydraulically actuated fuel injectors is discussed in co-owned U.S. Pat. No. 5,826,562 to Chen et al. This patent recognizes that some front end rate shaping, such as ramp and boot shapes, can be accomplished by initially exposing only a portion of the piston to the high pressure working fluid during an injection event, and then later exposing its complete hydraulic surface to the working fluid pressure during the main portion of an injection event. In a typical example of a rate shaped fuel injector of this type, the piston and its bore are modified to include concentric step portions. When the piston is in its retracted position immediately proceeding an injection event, only a central relatively small area portion of the piston is exposed to the working fluid pressure. After the piston has moved through an initial portion of its downward stroke, its central top hat portion clears a small diameter portion of the piston bore to expose the complete upper hydraulic surface of the piston to the working fluid pressure. Thus, when in operation, the piston initially moves relatively slowly to produce a relatively low injection rate and then later during its stroke it accelerates for the main injection event at significantly higher injection rates. While this rate shaping strategy has proven successful, there remains room for improvement.
In order for a stepped top piston to reliably produce rate shaping, the relatively large shoulder hydraulic surface area of the piston is preferably exposed to a known and relatively constant low pressure during the initial stroke of the piston. If the fluid pressure on the outer shoulder area of the piston can not be maintained at a relatively low known pressure during the initial portion of the injection event, then little or no rate shaping can be accomplished. Because the volume above the shoulder area of the piston must necessarily grow as the piston moves during its downward stroke, there must be some means provided for channeling fluid into this space in order to allow the piston to move in a known manner without being inhibited by vacuum effects or damaged due to a possible cavitation effects. Because fluid flow to the shoulder area is at least partially a function of a diametrical clearance between the top hat portion of the piston and its small diameter piston bore, some variation between injectors is possible due to the necessity to accept realistic machining tolerances on the two separate components. Thus, while the rate shaping concept has been proven successful, there remains room for improving the consistency between multiple injectors. In other words, there remains room for decreasing performance variations between injectors at least in part by decreasing the sensitivity of injector performance to dimensional variations in mass produced parts that are a necessity in almost any mechanical multi-component mechanical device.
The present invention is directed to overcoming these and other problems and to improving upon the predictability of injector performance and to decreasing variations in performance from one injector to another.
A fluid driven piston assembly comprises a body that defines a piston bore, a low pressure area and an actuation fluid passage. The piston has a hydraulic surface and is positioned in the piston bore. It is moveable a stroke distance between a retracted position and an advanced position. The hydraulic surface can be divided into a first hydraulic surface and a second hydraulic surface. The first hydraulic surface is exposed to fluid pressure in the actuation fluid passage over the stroke distance, but the second hydraulic surface is exposed to fluid pressure in the low pressure area over an initial portion of the stroke distance. The second hydraulic surface is exposed to fluid pressure in the actuation fluid passage over a different portion of the stroke distance.